Boundary condition at interface of two different materials in structural analysis module

Hi,

I am trying to simulate a structural mechanics problem that utilizes 3D solid stress-strain module. I have a square plate of elastomeric material having a thikness in micro meteres and on top of this elastomeric substrate I have a metalic pattern having length in cm and thickness and width in micro meters. I want to know what B.C I should use on the interconnect face that is directly in contact with the surface of the substrate. Actually there are two options free and fixed since there are no continuity conditions on interfacial boundaries in structural mechanics module.

I shall be obliged if any one can give some guidance over this issue.

Thanks